There are two solutions to this:
Solution 1:
Try some of the backdoor passwords that are often built into the BIOS following the setups on this site.
Solution 2:
- Turn off and unplug the computer
- Open the computer and look for a flat "watch" battery on the mother board
- Remove this battery. You may have to gently pry it out using a small tool.
- Plug your computer in and turn it on without the better.
- Turn your computer off and reinsert the battery and turn the system back on.
You will likely have to set the time and date in your bios.